It is postulated that the C equals O groups of phospholipids mediate the penetration of membranes by water, cations, carbohydrates, and proteins, and that membranes are closed by anhydrous hydrogen bonding of the C equals O groups in which the beta-hydroxyl of cholesterol serves as the hydrogen bond donor. This theory also explains the role of sphingolipids, plasmalogens and alpha-hydroxy acids in membranes. To test the theory, a number of phospholipids and phospholipid analogs will be synthesized which have two, one, or no C equals O groups, or a C equals O group in either position 1 or position 2 of the glyceride. The influence of these structures on membrane permeability will be measured, alone and in combination with cholesterol. Sphingolipids and plasmalogens will also be tested. The experiments will be performed on monolayers (cholesterol-phospholipid condensation effect; penetration by proteins) and on lipid vesicles (liposomes), in which potassium leakage and swelling by glycerol or glucose will be investigated. The penetration of lipid double layers by hydrogen bond donors and acceptors, and its regulation by cholesterol, will also be studied.